1. Technical Field
The present invention relates in general to spindle motors for disk drives, and, more specifically, to ball bearings and oil bearings for such spindle motors. More specifically, the present invention relates to disk drive ball bearing and oil bearing spindles containing antistatic additives.
2. Description of the Related Art
Computer disk drives, such as hard disk drives, use a motor to turn a magnetic platter at high speeds. The motor typically includes a spindle, which is made of a number of metallic materials, plastics and elastomers, that requires lubrication during operation. One solution is to use oil bearings. These oil bearings typically include a pair of magnetic fluid seals, several capillary holes or channels, and a magnetic fluid for lubrication, which flows within these capillary channels and is held in place by the magnetic fluid seals. An alternative spindle bearing system uses grooved hydrodynamic bearings lubricated by a low viscosity hydrocarbon fluid. The more conventional spindle designs utilize two ball bearings usually straddled across either side of the motor stator that is press fit onto the shaft. Unfortunately, the fluids usually lack adequate conductive properties and static charge typically builds up that can ruin the head disk assembly, or read or write heads used in the hard disk devices.
For example, a voltage difference between the heads and disks can provide a potential energy that can harm the sensitive components in the disk drive assembly, such as the magneto resistive (MR) heads and the disk media itself. Although the initial voltage differences can be kept to near to zero, certain charging mechanisms inside the head disk assembly (HDA) allow voltage differences to build up.
One example of static build up occurs at two important interfaces inside the HDA. The first interface is between the heads and the disk. The second interface is between the moving and stationary parts of the spindle bearing system itself. When surfaces rub against each other in a bearing, some electron and ion exchange or charge separation is expected. This is known as tribocharging. Even occasional contact of asperities on the heads and disks can generate this tribocharge. Due to the very close proximity of surfaces in a disk drive platter, even a small charge imbalance forms potential differences on the order of volts, which is sufficient enough to destroy the HDA.
Additional methods by which damaging overcharge occurs is by the shearing of air molecules in the boundary layer adjacent to the rotating surfaces. Bonds are broken at the lubricated interfaces in the spindle bearing system, which is further degraded by the continuous charge separation occurring inside the HDA. From a design standpoint, charging rate and the impedance across the bearings, both head-disk air bearing and the spindle bearing, govern the system performance. A high charging rate is dangerous when the impedance is high because of build up that is sufficient in its potential difference to ruin the HDA. Accordingly, it is known to keep the impedances as low as possible when designing the spindle bearings.
The grease and the oil used in the bearings typically are dielectric. Due to the charging that takes place as described above, charge accumulation takes place at the heads, or spindle, and disk platters, which generates an electrical potential difference between the heads and the disk platters. The air gap between the head and disk platter is on the order of tens of nanometers, so a small electrical potential difference results in a significant electric field gradient. When a sufficient electric field gradient exists, the charge dissipates across the air gap between the head and the disk. The discharge damages the read/write electronics.
In the case of an oil bearing spindle design, one solution to provide protection against the static build up is to have separate electrically conductive ferrofluid seals, in which the fluid is electrically conductive. Unfortunately, the ferrofluid seals require a considerable amount of space in the spindle cavity and do not provide reliable low spindle resistance. Additionally, as the form factor size of disk drive systems gets smaller and smaller, the technology reaches the limitations of the ferrofluid seal solution and so an alternative solution is necessary.
Accordingly, what is needed is a spindle motor assembly that uses an oil bearing having sufficient antistatic properties so as to provide sufficient conductivity for dissipating the charge through the oil rather than through the head disk interface. What is also needed is a method of overcoming the problems inherent with the ferrofluid bearing solution. Alternatively, what is needed is a spindle motor assembly that uses a ball bearing with sufficiently enhanced conductivity by virtue of using grease having sufficient antistatic properties.